This letterbox is located at the Aquatic Park
Park near Bolivar Street.

Follow the paved path to the left, along the water. You’ll pass a playground, and a number of disc golf chain baskets. Near basket 14, look for two posts by the water. The unpainted post should have a tuft of grass growing in it. That's mighty odd.

As a final note, I would like to mention that this box is very near Another Snowy Egret. If you've made the trip, why not swing by and get that box, too? And, of course, I don't need to tell you that this park is swarming with people, and that you should use discretion when retrieving and replacing this letterbox.